The present invention relates to a process for making bubble-free synthetic quartz glass.
Quartz glass has excellent properties such as excellent transmittance to light having a broad range of wavelengthsxe2x80x94from ultraviolet to infrared light, a very low thermal expansion coefficient and excellent heat and chemical resistance. Quartz glass is thus indispensable in a number of applications associated with very large scale integration (VLSI) chip fabrication, including wafer boats, furnace process tubes, deep-uv optics and mask substrates.
Known methods for manufacturing quartz glass include production by high-temperature vitrification and fusion using natural quartz as the raw material, a direct process involving the continuous production of a quartz glass ingot from a chemically synthesized silicon compound by high-temperature oxidation or hydrolysis, an indirect process wherein a chemically synthesized silicon compound is converted by high-temperature oxidation or hydrolysis into a very fine silicon dioxide aggregate known as xe2x80x9csootxe2x80x9d which is subsequently vitrified, and a sol-gel process in which silica gel is prepared by a sol-gel reaction, then sintered and vitrified at an elevated temperature.
Quartz glass employed in optical elements and mask substrates is required to have optical uniformity and high light transmittance. Hence, use is primarily made of synthetic quartz glass manufactured by a direct or indirect process involving continuous production of a quartz glass ingot by the high-temperature oxidation or hydrolysis of a chemically synthesized silicon compound.
The direct process involves burning oxygen and hydrogen, for example, to produce a high-temperature flame, and heating a quartz ingot with the high-temperature flame. A silicon compound serving as the silicon source is introduced into the flame, converting the silicon to silicon dioxide, which is at the same time continuously deposited onto the surface of the ingot, thereby effecting synthetic quartz glass growth. In this process, the silicon raw material is oxidized or hydrolyzed in a high-temperature oxyhydrogen flame, yielding an extremely fine quartz material known as soot. This quartz is vitrified by the elevated temperature generated at the oxyhydrogen flame, following which it is deposited and fixed to the ingot, resulting in continuous growth of the ingot.
However, not all of the soot comes into contact with and deposits onto the ingot. Finely divided silicon oxide which does not contact and settle onto the region of growth on the ingot is discharged as free soot from the reaction and growth system, becoming suspended soot. Free soot coalesces inside and outside of the flame into larger-size material which may find its way back into the flame and settle on the regions of ingot fusion and growth. Because the soot is large in size, it does not fully melt in the ingot fusion region, and so ends up remaining as unfused areas. Such places become the starting point for bubble formation within the ingot. As ingot growth proceeds, the bubbles grow to an enormous size.
If heat-resistant substances such as mineral substances or metal dust are present in the atmosphere near the flame or near the ingot growth region, they are drawn into the flame or ingot fusion region and reach the ingot growth region, becoming foreign inclusions which trigger bubble formation and lower the homogeneity of the synthetic quartz glass. Because a large amount of such mineral substances, metal dusts and the like are present in air as dust and particulates, the synthetic quartz glass must be grown in an apparatus capable of providing a clean process atmosphere shut off from normal outside air. In a conventional arrangement typically adopted for this purpose, the synthetic quartz glass production apparatus is enclosed in a chamber, clean air obtained by passing outside air through a filter to remove dust and particulates is led into the chamber, and suspended coalesced soot which has not taken part in growth of the quartz glass is rapidly discharged outside of the reaction system.
However, in addition to the clean air mentioned above, there exist within the synthetic quartz glass production apparatus gas streams of differing velocities and directions, such as a high-speed oxyhydrogen flame gas stream and a hot gas stream from the burner. Turbulence by these streams causes some of the free suspended coalesced soot to settle on the inside walls of the apparatus near the growth region instead of being discharged outside the reaction and growth system. Moreover, turbulence of the gas streams and an increased amount of deposition cause this soot to be re-suspended so that it ultimately reaches the ingot fusion region, where it triggers bubble formation.
Problems of this type have not been limited only to the direct process described above, but have similarly arisen in other synthetic quartz glass production processes.
It is therefore an object of the present invention to provide a process for producing bubble-free synthetic quartz glass which is able to reliably discharge from the system suspended soot that has not been fixed as synthetic quartz, and can thus prevent the suspended soot from re-settling in the region of ingot growth.
We have discovered that if, in a synthetic quartz glass manufacturing process, a finely divided quartz soot-forming high-temperature gas zone within a synthetic quartz glass production chamber has a suspended soot-discharging gas passed therethrough in a state that is flow-straightened in the direction of suspended soot discharge, and if the flow-straightened gas is preferably passed through the high-temperature gas zone in a direction within preferably xc2x115 degrees of the direction of flow by a stream of high-temperature gas directed at the high-temperature gas zone, re-settling onto the ingot growth region of the suspended soot that has not been fixed as quartz can be prevented, making it possible to obtain bubble-free synthetic quartz glass.
Accordingly, the invention provides a process for manufacturing synthetic quartz glass comprising the steps of feeding a quartz glass-forming raw material to a high-temperature gas zone within a chamber, converting the quartz glass-forming raw material into quartz soot, forming synthetic quartz glass from the soot, and flowing a gas through the chamber in the vicinity of the high-temperature gas zone for discharging suspended soot in a direction, the discharging gas being flow-straightened in the suspended soot discharging direction.
Preferably, a stream of high-temperature gas flows through the high-temperature gas zone in a direction, and the flow-straightened suspended soot-discharging gas flows through the chamber in the vicinity of the high-temperature gas zone in a direction within xc2x115 degrees relative to the flow direction of the high-temperature gas stream.